Typically, variable pitch aircraft propeller systems include: a plurality of propeller blades extending radially from a central hub, an engine for rotating the hub and propeller blades, and a pitch actuating system that pivots the propeller blades about their longitudinal axes to vary the angle (pitch) of the blades with respect to the approaching air flow. The hub has an interior chamber which houses the pitch actuating system.
Pitch actuation systems generally include a pitch lock. In the event of a malfunction of the pitch actuation system, the pitch lock prevents overspeed of the engine by preventing the blade from retarding to a lower pitch.
One type of pitch lock is hydromechanically controlled. A pressurized fluid holds the pitch lock, which is spring-loaded, out of locking engagement. Upon a determination that the propeller blade is undesirably moving toward a lower pitch, the pressurized fluid is bled away allowing the spring to urge the pitch lock into engagement with stationary pitch lock teeth thereby locking the movement of the blade. Examples of this type of pitch lock include U.S. Pat. Nos. 2,940,527 to H. L. Van Alstyne, et al. entitled "PROPELLER PITCH LOCK DISABLING AND FEATHER LOCK MECHANISM"; 2,985,244 to Fischer, et al. entitled "PROPELLER BLADE PITCH LOCK CONTROL MEANS"; 3,187,819 to Barnes, et al. entitled "COMBINED PITCH LOCK AND FEATHER LATCH", and 3,439,745 to Gaubis, et al. entitled "COMBINED PROPELLER PITCH LOCK AND LOW PITCH STOP SYSTEM".
Some pitch locks form part of an hydraulic actuator. For instance, U.S. Pat. Nos. Re. 26,461 to Barnes, et al. entitled "PITCH CONTROL SYSTEM", and 4,523,891 to Schwartz, et al. entitled "PROPELLER PITCH CHANGE ACTUATION SYSTEM", show a screw attaching to an hydraulic actuator that prevents the propeller blade from moving to low pitch.
Some purely mechanical (as opposed to hydraulic) pitch actuators utilize fully mechanical pitch locks. For instance, U.S. Pat. Nos. 2,856,012 to Frankland entitled "MECHANICAL PITCH LOCK", and 3,895,884 to Andrews entitled "TORQUE SENSITIVE PITCH LOCK" each show a splined ring having teeth that force an engaging ring into a stationary lock ring when a propeller blade is undesirably moving toward a lower pitch.
Other means and apparatus are sought to prevent a propeller blade from changing its pitch to prevent overspeed conditions. It is to that end that this invention is directed.